Three things we
thought we understood about shale gas, but were afraid to ask…
Cluff, Robert M.1,
Keith W. Shanley1, Michael A. Miller2 (1) The Discovery
Group, Inc, Denver, CO (2) BP America Production Co, Houston,
Shale-gas plays are largely evaluated and
interpreted on the basis of performance data from a limited number of field
areas supplemented with even more limited core data. The Barnett Shale at
Newark East Field in the Ft. Worth Basin is the standard by
which most shale plays are compared. We suggest this leads to a highly skewed
view of these play-types and that three fundamental areas of uncertainty exist
requiring additional research. (1) Significant shale-gas production is thought
to occur at thermal maturities coincident with the gas window. This reflects
the drop in productivity in the Barnett Shale associated with an Ro of 1.2%. Explanations offered are (1) insufficient gas
generated at lower maturity levels, and (2) oil in the pore structure blocking
the movement of gas. Laboratory data and modeling suggest gas is co-generated
with oil and low-maturity shale plays show shales can
be gas-saturated at the base of the oil-window. The idea that oil blocks gas is
based on relative permeability analogies to conventional reservoirs yet realisitic relative permeabilities
have not been measured in shales. (2) We think we
understand the partitioning between free and adsorbed gas by subtracting
helium-derived porosity from a total sorption isotherm. Helium is much smaller
than methane so helium porosity greatly exceeds the space available to methane.
The adsorbed-fraction is underestimated and porosity is overestimated. Because
pore-sizes are much greater in conventional reservoirs this problem was not
previously encountered. (3) Shales are viewed by many
as extremely tight sandstones where mass transport through micropores
and pore-throats move gas from matrix to fractures to well-bore. Matrix permeabilities of shales range
from 500x10-6 mD to 1x10-9 mD. Effective permeabilities are almost certainly much lower.